The production of shrink-resistant polyester threads, which also have high tensile strength and are stretch-resistant for industrial purposes, is known. Thus, GB-A-1 325 297 describes a method for spinning, stretching and thermofixing in one work cycle. For reduction of the shrinkage up to 1% by weight branching components are incorporated besides thermal fixing. The branching components not only prevent stretching of the threads, but also reduce the strength and the modulus and thus the dimensional stability. In this method, there results a thermal shrinkage of more than 7% , measured at 160.degree. C., with a maximum breaking strength of 80 cN/tex with a titer of approximately 1100 dtex. However, a yarn with such a high tendency toward shrinkage is no longer sufficient for the requirements for the dimensional stability of a tire. In this case, breaking strength, thermal shrinkage and initial modulus of elasticity (Young's modulus of elasticity) must be coordinated with one another within determined limits.
But another method is known for production of chemically modified, quick spun threads (DE-B-1266992). However, in this method only the dyeing properties are characterized by means of the addition of chain branching agents or cross-linking agents such as polyalcohols in quantities of 0.10 to 1.0 moles %.
It has now been found that the dimensional stability of a tire can be expressed by means of a characteristic number if the essential parameters, such as breaking elongation or strength (Ft), initial modulus of elasticity (Mo) and thermal shrinkage (TS), are kept within determined limits. The following characteristic number is suggested for the evaluation of a dimensionally stable yarn with low shrinkage and a high initial modulus of elasticity: ##EQU1##
It has not been possible thus far to achieve a low shrinkage at a given reference elongation or strength (Ft) and a high initial modulus of elasticity through mechanical/thermal means.